Conventionally, as a monitoring apparatus in medical spots, a magnetic resonance imaging (MRI) apparatus 1 shown in FIG. 24 has been used (see, for example, Japanese Patent No. 3135592).
The magnetic resonance imaging apparatus 1 is an apparatus that forms gradient magnetic fields in X-axis, Y-axis, and Z-axis directions in an imaging region of a patient P, who is set inside a cylindrical magnet for static magnetic field 2 forming a static magnetic field, using respective gradient magnetic field coils 3x, 3y, and 3z of a gradient magnetic field coil unit 3 and transmits a radiofrequency (RF) signal of the Larmor frequency from a radio frequency (RF) coil 4 to thereby cause a nuclear spin in the patient P to magnetically resonate and uses a nuclear magnetic resonance (NMR) signal caused by excitation to reconstruct an image of the patient.
A static magnetic field is formed inside the magnet for static magnetic field 2 by a static magnetic field power supply 5 in advance. A sequence controller control unit 7a gives a sequence, which is control information for a signal, to a sequence controller 8 according to a command from an input device 6. The sequence controller 8 controls a gradient magnetic field power supply 9, which is connected to the respective gradient magnetic field coils 3x, 3y, and 3z, and a transmitter 10, which gives a radiofrequency signal to the RF coil 4, in accordance with the sequence. Therefore, a gradient magnetic field is formed in the imaging region and the radiofrequency signal is transmitted to the patient P.
In this case, the X-axis gradient magnetic field, the Y-axis gradient magnetic field, and the Z-axis gradient magnetic field formed by the gradient magnetic field coils 3x, 3y, and 3z are mainly used as a phase encoding (PE) gradient magnetic field, a readout (RO) gradient magnetic field, and a slice encoding (SE) gradient magnetic field. Consequently, an X coordinate, a Y coordinate, and a Z coordinate, which are position information of an atomic nucleus, are transformed into a phase and a frequency of a nuclear spin and a position of a slice. The sequence is executed repeatedly while a phase encode amount is changed.
The NMR signal, which is generated according to the excitation of the nuclear spin in the patient P, is received by the RF coil 4 and given to a receiver 11 to be converted into digital raw data. The raw data is taken into the sequence controller control unit 7a via the sequence controller 8. The sequence controller control unit 7a arranges the raw data in a K space (a Fourier space) formed in a raw data database 7b. Then, an image reconstructing unit 7c executes the Fourier transformation on the raw data arranged in the K space, whereby reconstructed image data of the patient P is obtained and stored in the image data database 7d. Moreover, image data is given to a display device 7f and displayed by a display processing unit 7e according to circumstances.
In such a magnetic resonance imaging apparatus 1, for speedup of imaging, the RF coil 4 includes a whole-body (WB) coil for transmission and a phased-array coil (PAC) serving as a main coil for reception (see, for example, Roemer P B, et al. The NMR Phased-array, MRM 16, 192-225 (1990)). The phased-array coil includes plural surface coils. Thus, the phased-array coil can reduce imaging time by receiving NMR signals using the respective surface coils simultaneously and collecting a large number of raw data in a short time.
Here, in image diagnosis by the magnetic resonance imaging apparatus 1, it is desired that luminance unevenness (signal intensity unevenness) does not occur in image data that is obtained finally. However, when the RF coil 4 is constituted by the phased-array coil including the plural surface coils, because of non-uniformity of sensitivities of the respective surface coils constituting the RF coil 4, non-uniformity occurs not only in signal intensities of the NMR signals but also in signal intensities of the image data simply obtained by the reconstruction processing according to the Fourier transformation of the raw data. Thus, luminance unevenness occurs in the image data.
Thus, conventionally, a sensitivity pre-scan is executed prior to a main scan to generate an image of the patient P. Then, image data are acquired from the phased-array coil and the WB coil by the sensitivity pre-scan. A sensitivity distribution of the phased-array coil is estimated as three-dimensional sensitivity map data on the basis of a signal intensity ratio (SPAC/SWB), which is a division value of signal intensities SPAC and SWB of the respective image data, by a procedure of a flowchart shown in FIG. 25. Luminance of the image data is corrected by the obtained three-dimensional sensitivity map data.
First, a sequence for sensitivity estimation is given to the sequence controller control unit 7a by the sensitivity pre-scan execution unit 7g and sensitivity pre-scan is executed. A WB reconstructed image obtained by the WB coil and a PAC reconstructed image obtained by the phased-array coil are stored in a WB reconstructed image database 7h and a PAC reconstructed image database 7i, respectively.
An estimated value of a sensitivity distribution of the phased-array coil is calculated by a sensitivity distribution estimating unit 7j on the basis of the WB reconstructed image and the PAC reconstructed image.
In step S1, threshold value processing is executed on the WB reconstructed image and the PAC reconstructed image by a threshold value processing unit 7k. In other words, regions, where signal intensities of the WB reconstructed image and the PAC reconstructed image are equal to or lower than a threshold value, are masked and WB absolute value image data and PAC absolute value image data are generated.
Next, in step S2, a region reduction unit 71 reduces regions after the threshold value processing of the PAC reconstructed image and the WB reconstructed image according to region reduction processing and excludes sections with small signal intensities near the mask regions from data for three-dimensional sensitivity map generation.
Next, in step S3, a division processing unit 7m divides a PAC absolute value image (a signal absolute value of the PAC reconstructed image) in each slice after the threshold value processing and the region reduction processing by a WB absolute value image (a signal absolute value of the WB reconstructed image) to thereby calculate a signal intensity ratio of the PAC absolute value image and the WB absolute value image as three-dimensional sensitivity map data.
Next, in step S4, normalization processing for the three-dimensional sensitivity map data, which is calculated as the signal intensity ratio of the PAC absolute value image and the WB absolute value image, is carried out by a normalizing unit 7n for each slice.
Next, in step 5, a patient inner region interpolating unit 7o applies linear interpolation processing to a no-signal region in the patient P of the three-dimensional sensitivity map data after the normalization processing.
Next, in step S6, in order to estimate a sensitivity distribution in a no-signal region outside the patient, region growing processing is carried out by a patient outer region interpolating unit 7p and a sensitivity distribution of the no-signal region outside the patient is interpolated.
Next, in step S7, a slice direction weighting unit 7q corrects the three-dimensional sensitivity map data by weighting the three-dimensional sensitivity map data in a slice direction such that a sensitivity distribution in the slice direction is made uniform.
Next, in step S8, various kinds of processing such as fitting processing and smoothing processing are carried out by a smoothing processing unit 7r. A sensitivity map is created as volume data in an entire three-dimensional region and stored in a sensitivity map database 7s. 
Further, an image acquiring sequence is given to the sequence controller control unit 7a by a main scan execution unit 7t and a main scan is executed. Then, image data is obtained by image reconstruction processing of the image reconstructing unit 7c and stored in the image data database 7d. Moreover, an image data correcting unit 7u corrects luminance of the image data stored in the image data database 7d using the sensitivity map stored in the sensitivity map database 7s. Image data after the luminance correction is given to the display device 7f and displayed by the display processing unit 7e. 
In general, a no-signal region is included in image data obtained by the sensitivity pre-scan and the main scan. This is because regions such as lungs are present in a patient forming an imaging region and a no-signal region, where an NMR signal is not generated, is present. Here, in general, a phenomenon in which a signal intensity decreases occurs in a signal region near a boundary of a no-signal region and a signal region in a patient. This affects an estimated value of a sensitivity distribution of the phased-array coil.
However, in the conventional magnetic resonance imaging apparatus 1, in the method of interpolating a no-signal region in creating a sensitivity map of the phased-array coil, the region growing processing is only carried out for the no-signal region regardless of the inside and the outside of the patient.
Moreover, the sensitivity pre-scan using the phased-array coil and the sensitivity pre-scan using the WB coil are carried out separately. Thus, it is likely that a gap occurs between a shape of the patient P in the image data acquired by using the phased-array coil and a shape of the patient P in the image data acquired by using the WB coil.
In addition, non-uniformity occurs in signal intensities in a Z-axis direction of a device coordinate system because of not only a sensitivity distribution but also an arrangement characteristic of the respective surface coils of the phased-array coil.
However, in the conventional magnetic resonance imaging apparatus 1, non-uniformity of signal intensities due to the misregistration of image data in the sensitivity pre-scan and the arrangement characteristic of the surface coils is not taken into account. A sensitivity distribution of the phased-array coil is estimated on the basis of a signal intensity ratio of the PAC absolute value image data and the WB absolute value image data, which are obtained simply by applying the threshold value processing to the respective signal intensities of the WB reconstructed image data and the PAC reconstructed image data, respectively, and a sensitivity map is generated.
As a result, in the conventional magnetic resonance imaging apparatus 1, a problem occurs in that accuracy of the sensitivity distribution of the phased-array coil estimated by the sensitivity pre-scan is not obtained sufficiently and luminance of the reconstructed image obtained by the main scan cannot be corrected at sufficient accuracy.
In addition, conventionally, as a monitoring apparatus in medical spots, a magnetic resonance imaging apparatus 1 shown in FIG. 26 is also used (see, for example, Japanese Patent No. 3135592).
As described above, when the RF coil 4 is constituted by the phased-array coil and the WB coil, because of non-uniformity in sensitivities of the phased-array coil and the WB coil, non-uniformity occurs not only in signal intensities of NMR signals but also in signal intensities of image data obtained by the reconstruction processing. In general, non-uniformity in sensitivities of the WB coil is so small as to be negligible. However, in particular, non-uniformity in sensitivities of the surface coils in the phased-array coil serving as coils adapted to respective purposes is large, which affects image data.
Therefore, it is necessary to correct the non-uniformity in signal intensities in image data due to the non-uniformity in sensitivities of the phased-array coil.
Thus, in the magnetic resonance imaging apparatus 1 shown in FIG. 26, as shown in a flowchart in FIG. 27, in step S1, a sequence for sensitivity estimation is given to the sequence controller control unit 7a by a sensitivity pre-scan executing unit 7g and sensitivity pre-scan is executed using the phased-array coil and the WB coil as reception coils. Then, WB coil image data obtained by the WB coil and main coil image data obtained by the phased-array coil are acquired as image data for estimating a sensitivity distribution of the phased-array coil and stored in a WB coil image database 7h and a main coil image database 7v, respectively. Therefore, imaging of volume data, which is three-dimensional image data, is carried out twice.
Next, in step S2, an estimated value of a sensitivity distribution of the phased-array coil is calculated by a sensitivity distribution estimating unit 7j. A signal intensity SPAC of main coil image data shown in (a) in FIG. 28 is divided by a signal intensity SWB of WB coil image data shown in (b) in FIG. 28 by a division processing unit 7w, whereby a signal intensity ratio (SPAC/SWB) of the main coil image data and the WB coil image data shown in (c) in FIG. 28 is calculated as an estimated value of a sensitivity distribution of the phased-array coil.
In this case, in order to prevent division processing from being applied to regions where the signal intensities SPAC and SWB of the main coil image data and the WB coil image data are less than a threshold value, for example, less than 10% of a maximum value, threshold value processing for the respective signal intensities SPAC and SWB is carried out by a threshold value processing unit 7x as pre-processing of the division processing as required. The respective signal intensities SPAC and SWB in the regions, where the signal intensities SPAC and SWB of the main coil image data and the WB coil image data are less than the threshold value, are masked.
Influence of image data on signal intensities, which is caused by factors other than non-uniformity in a sensitivity distribution of the phased-array coil such as an image contrast, is cancelled by the division processing having this threshold value processing as pre-processing. Thus, it is possible to estimate a sensitivity distribution at satisfactory accuracy.
Next, interpolation or extrapolation is applied to a no-signal region, which is a data absence section caused by the threshold value processing because of presence of the lung fields or the like, by an interpolation unit 7y to estimate a sensitivity distribution. Further, fitting processing and smoothing processing are carried out over an entire two-dimensional region by a smoothing processing unit 7z to obtain an estimated value curve of the sensitivity distribution shown in (d) in FIG. 28.
Then, the same processing for image data is carried out over respective sections of an entire three-dimensional region to obtain an estimated value of the sensitivity distribution as volume data.
Next, in step S3, the estimated value of the sensitivity distribution of the phased-array coil is stored in the sensitivity map database 7s as three-dimensional sensitivity map data.
Next, in step S4, a sequence for image acquisition is given to the sequence controller control unit 7a by the main scan executing unit 7t and main scan is executed using the phased-array coil as a reception coil. Then, row data is collected and image data is obtained by image reconstruction processing of the image reconstructing unit 7c. 
Next, in step S5, according to conditions such as an imaging condition including an imaging sectional direction and spatial resolution, a data collection condition, and an image reconstruction condition in the main scan, an image data correcting unit 7u slices corresponding three-dimensional sensitivity map data from the sensitivity map database 7s. 
Then, in step S6, the image data correcting unit 7u corrects the image data using the sliced three-dimensional sensitivity map data. Consequently, non-uniformity in signal intensities of the image data is corrected.
On the other hand, a method of estimating a sensitivity distribution of the phased-array coil as sensitivity map data from image data itself, which is obtained from NMR signals received by the phased-array coil, according to post-processing and correcting signal intensity unevenness of the image data using the obtained sensitivity map data of the phased-array coil is also used. For example, there is a method of generating image data of an extremely low frequency component by executing smoothing processing on the image data obtained by the phased-array coil and substituting the image data for a sensitivity distribution.
In addition, there have been proposed a technique for correcting signal intensity unevenness of image data, which is obtained from NMR signals received by a phased-array coil, with reference to a signal intensity of a radiofrequency signal transmitted from a WB coil (see, for example, JP-A-63-132645) and a technique for correcting signal intensity unevenness of image data using a sensitivity distribution of a phased-array coil stored in advance and position information of the phased-array coil obtained by estimation from image data (see, for example, JP-A-7-59750).
However, in the conventional method of estimating a sensitivity distribution of a phased-array coil on the basis of a division value (SigPAC/SigWB) of signal intensity values SigPAC and SigWB of image data obtained from a phased-array coil and a WB coil by sensitivity pre-scan and correcting signal intensities of the image data, there is a problem in that the time required for the sensitivity pre-scan is long. Therefore, for example, when an abdomen of a patient P is imaged, breath-holding time is extended according to execution time of the sensitivity pre-scan.
In addition, it is likely that misregistration in the patient P occurs between collection of image data using the phased-array coil and collection of image data using the WB coil because of a cause such as movement of the patient P or the like. Moreover, in order to acquire image data from both the phased-array coil and the WB coil, decoupling between the phased-array coil and the WB coil is required to be complete.
On the other hand, in the method of correcting signal intensity unevenness of image data using a sensitivity distribution of a phased-array coil estimated by post-processing from the image data itself obtained by the phased-array coil, correction of signal intensities of the image data is insufficient because accuracy of estimation of a sensitivity distribution of the phased-array coil is low. Thus, it is impossible to obtain uniformity of image data after correction, which is obtained finally, sufficiently.
There is also a problem in that it is difficult to always apply correction to various image types at sufficient accuracy. For example, when image data is image data having a desired contrast like T1 weighted image data and T2 weighted image data, even if the smoothing processing is applied to the image data to estimate a sensitivity distribution, image data showing the sensitivity distribution also has a contrast. Thus, it is impossible to use image data after the smoothing processing as a sensitivity distribution.
Moreover, it is difficult to consistently perform correction of image data in all slices in imaging by multi-slice.
The invention has been devised in order to cope with such conventional circumstances and it is an object of the invention to provide a magnetic resonance imaging apparatus that is capable of estimating a sensitivity distribution of an RF coil accurately on the basis of image data obtained by implementation of sensitivity pre-scan and correcting luminance of image data obtained by implementation of main scan satisfactorily on the basis of the obtained sensitivity distribution of the RF coil and a data processing method for the magnetic resonance imaging apparatus.
It is another object of the invention to provide a magnetic resonance imaging apparatus that is capable of correcting signal intensity unevenness of image data due to non-uniformity in a sensitivity distribution of a reception coil in a shorter time and at satisfactory accuracy regardless of an imaging condition such as image types and a data processing method for the magnetic resonance imaging apparatus.